Lasers are a common power source for material processing and metal additive manufacturing, such as laser additive manufacturing (LAM) technologies. As one example, metal powder bed LAM involves a manufacturing platform or bed that can be raised and lowered during the manufacturing process. A thin layer of metal powder is evenly spread across the bed, and then a laser is used to heat the metal powder in a desired pattern so that it melts and then cools, while the unaffected powder material can be brushed away, leaving only the newly formed layer. After each layer is formed by the laser, the powder platform is lowered and a new layer of metal powder is spread on top of the old layer. In this manner, a three-dimensional object can be formed, one layer at a time, by lowering the platform, adding a new powder layer, and then using the laser to melt the powder in the shape of a desired object volume into the new powder layer, where it then cools, consolidates into metal and bonds with the previous layer [1-4]. The major drawbacks of existing laser power sources for LAM are the lack of active and/or adaptive control of the laser beam spatiotemporal characteristics during laser energy deposition and lack of appropriate in situ sensing techniques for characterization of both stock material in front of the processing beam and melted and consolidated into metal materials inside the processing beam and the heat affected zone (HAZ), during and after LAM processing of each layer. The lack of such real-time sensing techniques prevents development and implementation of the beam control techniques including, programmable, feedforward and feedback control of LAM processes to improve productivity, repeatability and quality of LAM-built products and components [5].
It has also been found that the desired improvement of micro-structure and surface finish, mitigation of residual stress, and increase of processing speed are difficult to achieve with a single laser beam. The availability of advanced power sources and control systems disclosed herein, capable of simultaneously projecting multiple laser beams whose characteristics, such as optical power, focal spot size, pointing and steering characteristics, can be individually controlled, will create new opportunities for LAM.
Recent technology developments may indicate a trend towards examining the advantages of, and developing systems for, multi-beam controllable laser power sources for material processing and LAM. Currently, several dual-beam and four-beam laser systems adapted for laser material processing and LAM have been demonstrated [6-7]. The existing multi-beam LAM systems utilize separate optical trains for each beam composed of laser sources (100.1) that generates laser beams (100.2), beam forming (100.3), scanning (100.4), and focusing (100.5) optics. FIG. 1 illustrates a LAM system for independent processing of the stock material using N projected beams (100.6) that form focal spots (100.7) at the powder bed surface (100.8) or other point of manufacture work pieces. Additional beam combining optics (100.9) are required to LAM processing with co-located or closely located focal spots as illustrated in FIG. 2 for the case of N-beam LAM. Scaling of the existing multi-beam LAM systems as shown in FIG. 1 and FIG. 2 to include large numbers of individually controlled laser beams would require the integration of multiple optical trains similar to those shown in FIG. 1 and FIG. 2, resulting in an extremely bulky, heavy and expensive LAM system. In addition, the demonstrated multi-beam LAM systems do not include sensors for feedback control and thus cannot provide on-the-fly modification of laser beam characteristics based on work piece sensing data. The systems and methods disclosed herein offer solution to these problems.
Another major drawback of the existing LAM systems is that they are largely based on the so-called single-point-processing technique [5,9]. In the systems illustrated in FIG. 1 and FIG. 2, sharply focused projected laser beams (100.6) create highly localized (point) heat sources that are rapidly rastered (scanned) with beam scanning optics (e.g., galvo mirrors) for selective laser melting (SLM) of a stock material.
This single-point-processing LAM technique suffers from several major drawbacks:
A. A highly localized (point) heat source that is generated by a sharply focused laser beam at a powder bed or other manufacturing work piece, creates large thermal gradients in the processing material. Scanning of this point-heat source produces an elongated molten pool, which at high scanning speeds breaks into disconnected balls due to Rayleigh instability [10,11]. Both large thermal gradients and these balling effects negatively impact surface roughness, cause residual stresses and cracking in LAM, and limit productivity. Note that attempts to increase LAM productivity by using higher laser powers with faster scanning speeds could make surface finish and residual stress even worse [12];
B. In single-point processing, the laser beam spot diameter, ranging from about fifty to hundreds of microns, only marginally exceeds the characteristic powder particle size (˜10-45 μm for Ti-6Al-4V alloy [13]). The result is a tiny processing volume, containing a comparatively small number of powder particles of different sizes within the volume. Since laser beam absorptivity and the material's temperature rise is dependent on particle size, any variability of the stock material inside the small processing volume leads to anisotropy in heat dissipation, variations in local temperature gradients, and strong fluid flows in the molten pool [14-16]—all major factors that directly impact the quality of LAM-produced components; and
C. Processing with a single laser beam requires high-speed focal spot rastering (scanning) to avoid unacceptably long manufacturing times. This in turn results in extremely high heating rates leading to disruptions in the powder bed layer or material from evaporative flows, and from splatter due to evaporative recoil and jetting [5,16]. High heating rates also make it difficult, or even impossible, to achieve real-time sensing and control of LAM process parameters.
These drawbacks for current single-point LAM technology can be alleviated with systems and methods disclosed herein.
The most recent attempt to move beyond conventional single-point SLM is implementation of the additive manufacturing process known as Diode Area Melting (DAM) [17]. DAM uses an array of low-power individually addressable laser diode emitters for parallel stock material processing through the use of multiple laser spots. The DAM approach has several principle problems that prevents its transitioning from the current early stage lab experiments to the LAM industry. The large and highly asymmetric divergence of laser diodes results in elliptical poor-quality beams that are difficult to concentrate (focus) into a spot that has sufficient power density to cause the stock material to melt. To increase the power inside each individual laser spot, these diode stack arrays can in principle be combined. However, this multiplexing of laser sources complicates the focusing of these highly divergent beams even more [18]. In addition, the laser spot position on the powder bed surface or material cannot be individually controlled. This leads to a highly spatially non-uniform combined laser intensity with no ability to achieve adaptive spatiotemporal power shaping. The novel components, systems and methods disclosed herein offer solution to the problems discussed above as well as other problems present in conventional systems.